Potassium is one of the most frequently tested analytes in the clinical laboratory. Because of its critical role in body homeostasis, laboratory errors that cause inaccurate potassium results can significantly affect patient safety. This review is limited to the spurious increase of serum potassium levels (pseudohyperkalemia), ie, instances of elevated potassium results that cannot be explained clinically and do not correspond to the status of the patient; when these specimens are recollected, potassium values usually drop substantially without clinical intervention. Because the workup of falsely elevated potassium levels consumes valuable health care resources and can result in patient care delays, it is essential to identify all variables that can cause pseudohyperkalemia, understand the mechanisms by which these variables affect serum potassium levels, and define corrective actions to ameliorate the problem. In most cases, increases in serum potassium are due to factors in the preanalytic phase of the testing cycle. The effect of patient-specific variables and variables related to specimen acquisition and processing, handling, and transport are discussed.